Could you help me to identify monopoly, because I got confused. There is a company with market share of 54 %. The second one has market share of 21 %. Can this market be characterized as monopoly or maybe duopoly? Thanks in advance.
The definition of a monopoly is one company having the exclusive control of a good or service. Since there are 2+ companies it does not fit the definition of a monopoly.
A duopoly is a situation where 2 companies control the majority of the market. It sounds like this describes your situation.
A more correct term may be oligopoly which describes a market where a small limited number of entities controls the majority of the market. If there is a 3rd company which controls the remaining 25% market share this may be the most correct term but otherwise a duopoly is probably the correct definition.